1. Field of the Invention
This invention relates to a spark plug for an internal combustion engine suitable for a vehicle, a cogenerator, a gas transporting pump, or the like.
2. Description of the Related Art
A large engine, which is applied to a large generator using gaseous fuel (cogenerator), a gas transporting pump, or the like, usually includes a large spark plug having, for example, a housing outer diameter of 18 mm, an insulator head portion diameter of 14 mm, a central electrode diameter of 2.5 mm, and an entire length of 115 mm. This kind of large engine is always operated without intervals. Therefore, the large spark plug for the engine is especially required to have long life in order to improve a maintenance property and in order to reduce plug exchanging cost.
It is conceivable, to secure the long life of the spark plug, that a spark gap is prevented from being enlarged during spark discharge so as not to increase a required voltage necessary for the spark discharge. Therefore, as shown in FIG. 1, discharge members 25 made of a noble metal material having favorable expendability resistance are generally disposed on respective front ends of a central electrode 2 and an earth electrode 3, thereby preventing the spark gap from being enlarged.
The discharge members 25 having large areas, however, can lowers a spark discharge ignition property. The reason is considered as follows. That is, when the discharge members 25 have large areas to have specific calorific capacities, flame cores 7 produced by the spark discharge contact the discharge members 25 and are immediately cooled down. That is, the flame cores 7 are quenched without growing, resulting in deterioration of the spark discharge ignition property.
To solve this problem, JP-A-4-242090 proposes a noble metal chip as a discharge member that has a groove on a discharge face. However, in the spark plug, because the electrical insulating propery of the spark plug is so high that sparks are not liable to be produced, it is difficult to sufficiently comply with the above-described requirements of a large engine. In addition, it is easy for cracks to develop in the the noble metal chip due to thermal stress produced by a thermal expansion difference between the noble metal chip and the central electrode, so that it is easily separated from the central electrode, shortening the spark plug life.